The present invention relates to particulate refractory minerals which constitute the basis of synthetic molding sand used in metal casting and more particularly to refractory minerals which contain at least 85% silica.
The particulate refractory minerals having a composition of at least 85% silica are commonly designated as lake, bank, sharp, and silica sands. These refractory minerals are used in green sand molds. Green sand molding compositions for use in casting generally comprise silica of a size range of from 50 to 180 mesh, and clay such as sodium or calcium bentonite in quantities from 4.0 to 12.0 per cent by weight of the silica and sufficient water to render the silica and bentonite mass plastic and workable.
In the manufacture of green sand molds, it is required to maintain a relatively high grain packing density and simultaneously to maintain as high a degree of uniformity in density across the casting face of the mold as possible. Also the productivity of the foundry is increased by the rapidity at which molds can be produced.
To achieve the required mold characteristics and at the same time increase productivity the foundry industry are using hydraulically operated molding machines. These machines are constructed to apply very high pressure to compact the molding sand in the flask and against the pattern.
While these hydraulic machines have greatly increased the production of molds, many problems have also been created which are peculiar to high pressure compaction. One of the primary difficulties relates to the surface finish of the casting. Also there is a gross penetration of the refractory in the casting surface and a high gas back pressure at the mold surface. These two latter conditions also tend to deteriorate the surface finish of the casting.
It is the primary object of the present invention to utilize the advantages associated with high pressure compaction molding machines while minimizing the disadvantages encountered heretofore.
This is accomplished generally by altering the surface characteritics of the refractory silicate grains with a very thin coating of thermo-plastic resin. The coated grains are subsequently mixed with bonding clay and water to form a molding composition. The molding composition including the thinly resin coated silicate has improved response to compaction pressure.
A further object is to provide a mold composition including the resin coated silicate grains which completely separates from the metal casting at shakeout thereby to substantially reduce casting cleaning costs.
A further object is to provide a mold composition which permits the use of finer grains of the silicate than normally permissable so that the finer grains enhance the smoothness of the casting.
Another object is the provision of mold composition which includes a minimum quantity of carbonaceous material so that fine carbon dust build-up in used sand is minimized thereby to permit the molding sand to be reused with no increase in amount of water above the original level.
Still another object is to provide a mold composition in which the bonding force of the clay such as bentonite is used more effectively so that the quantity of clay required may be reduced 20 to 25% below the levels commonly used in high pressure molding.
In the more specific aspects of the invention, the mold composition of the present invention comprises a particulate refractory mineral having a minimum SiO.sub.2 content of 85% and a grain fineness number of 40-170 mesh, a bonding clay of about 1.5 to 8.0% by weight, and phenol formaldehyde thermoplastic resin of 0.5 to 5% by weight with sufficient water to render the entire mass plastic and moldable.